This invention relates to stents placeable in a lumen of the human body to maintain patency of the lumen.
The use of stents has become common in connection with procedures where it is desired to reinforce the wall of a vessel in the human body to maintain the patency of the lumen and reduce the risk of constriction of the lumen or collapse of the vessel wall. Stents have come into common use in connection with angioplasty procedures in which a blood vessel in the human body, having become obstructed, is dilated to restore the flow area of the lumen. A stent placed in the treated region of the lumen serves as a scaffold to support the vessel wall that defines the lumen. Stents may be placed as part of a surgical procedure or, as is more often the case, percutaneously, by navigating a slender catheter, on which the stent is mounted into and through the patient""s vasculature to the target site. Stents also are used in connection with other body lumens, such as in the urinary and biliary tracts, among others.
Most stents are generally tubular in shape and may be classified either as self-expanding or as balloon expandable. Self-expanding stents characteristically are in their expanded configuration when in a free, released state. In order to advance a self-expanding stent to the deployment site in the vessel, the stent is contracted to its small diameter (low profile) dimension and is mounted to the distal end of a delivery catheter that maintains the stent in its low profile configuration as the catheter is advanced to the target site. The stent is deployed by freeing the stent from the catheter to enable the stent to self-expand, under its inherent resilience, into supportive engagement with the vessel wall. The delivery catheter then can be removed from the patient, leaving the stent in place. In contrast, a balloon expandable stent does not rely on inherent resilience for its use or operation. Rather, a balloon expandable stent typically is formed as a metal tubular structure defined by a selected pattern of interconnected structures and links configured to enable the diameter of the stent to be expanded forcibly, as by a balloon, from its low profile diameter to a larger diameter at which it can support the vessel wall. During such expansion, the metallic stent undergoes plastic deformation and retains its expanded diameter. The balloon deployment catheter then can be deflated and withdrawn, leaving the stent in place. Another class of stents includes those formed from a shape memory alloy, such as a nickel-titanium alloy (nitinol). The alloy has a thermal-dependent memory in that it will maintain a stable configuration, as in a low profile configuration suitable for delivery by a catheter, but will return to its memory shape (e.g. to expand to a larger, vessel-supporting diameter) in response to a thermal event such as injecting warm saline into the vessel to trigger the shape memory phenomenon or exposing-the stent to body temperature. Such alloys also can be made to exhibit superelastic properties
Among the desirable features of a stent are that it should have sufficient hoop strength to support the vessel against the stresses that the vessel can be expected to impose on the deployed stent. The degree of scaffolding, a measure of the percentage of cylindrical area defined by the expanded stent as compared to the void spaces between its metal structures and links, should be selected to provide the desired balance between structural strength of the stent and exposure of the inner surface of the vessel to the lumen. It is important that the stent be capable of being contracted to a low profile diameter that is sufficiently small to facilitate percutaneous insertion of the stent and navigation of the stent through the sometime tortuous vasculature. The longitudinal flexibility of the stent also is an important characteristic, particularly in settings in which the stent must be navigated through tortuous vessels in order to reach the intended deployment site. Longitudinal flexibility also is important after the stent has been expanded in order that the natural curvature of the body lumen, which typically is deformed when the balloon is inflated, can return to its natural shape after the stent has been deployed and the delivery catheter removed. Also among the desirable characteristics of a stent is that it should have a sufficiently large expansion ratio, that is, the ability to expand from its low profile configuration to as large a diameter as can reasonably be expected, in order to treat the condition at hand. A large expansion ratio enables the physician to perform a procedure with an additional measure of confidence that if the original assessment of proper stent size is smaller than what is actually required, the stent can be expanded to a still larger diameter. Further, in many cases, it is important that the stent, when expanded radially, does not contract substantially, if at all, in length as it is deployed. Also among the desirable characteristics of a stent, particularly with balloon expandable stents, is that the expansion of the stent components be relatively uniformly distributed. Additionally, it is desirable that when the stent is expanded from its low profile to its deployed diameter, it does not have stress points that, upon expansion or after deployment, could lead to fracture of portions of the stent.
It is among the general objects of the invention to provide an improved stent construction and mode of operation by which the foregoing desirable characteristics of a stent may be obtained.
The invention is embodied in a tubular stent in which the wall of the stent is defined by a filigree-like pattern defined by regions of interconnected metal members and openwork. The pattern is characterized by a plurality of nodes arranged in clusters, each cluster comprising a group of nodes. Each of the nodes includes a central hub and at least three arms connected to and circumscribing, partially, the hub. When the stent is in its low profile, unexpanded state, each arm is arranged circumferentially about the hub of the node and lies closely adjacent to the next adjacent arm of the node in a spiral-like fashion. Each of the arms in a node is connected at a transition region to an adjacent node by being connected to the outer end of an arm of that adjacent node, the connected arms and transition region together defining a link between adjacent nodes. When the stent is expanded, as by a balloon catheter, the links uncoil. The links uncoil differentially, the degree to which a particular link uncoils depending on the degree to which the link is oriented in a generally circumferential direction. A link that is oriented in a generally circumferential direction may uncoil more than a link that extends in a direction that is closer to the axial direction of the stent. The nodes enable each of the links to unfold to the extent necessary to respond to the radial and axial forces applied to the stent during balloon expansion. The nodes are free to shift and reorient themselves in response to the applied forces of expansion.
One embodiment of the invention (FIG. 3) includes clusters that define a pattern of nodes arranged generally hexagonally. In another embodiment (FIG. 13), the nodes may be packed more closely together and are disposed to lie generally helically along the stent with each node along the helix being serially connected by a link to the next adjacent nodes on that helix.
In another aspect of the invention, the stent is formed by laser cutting the pattern of the stent from a metal tube having a diameter selected to correspond to the lowest profile that the stent is expected to have, typically the diameter that the stent will have when mounted and crimped onto the balloon of a delivery catheter. In order to load the stent onto the deflated balloon, the stent is preliminarily expanded slightly, for example, by advancing a tapered mandrel through the stent to increase its diameter sufficiently so that it can be slid onto the balloon. Once in position on the balloon, the stent then is crimped firmly about the balloon, with the stent returning toward its lowest profile configuration during the crimping process.
It is among the objects of the invention to provide an intraluminal stent that can be delivered while being maintained in a very low profile, yet in which the stent has a high degree of longitudinal flexibility to facilitate navigation through tortuous vessels; to provide an intraluminal stent that has sufficient flexibility in its expanded, deployed state to enable the vessel to return toward its natural shape; to provide such a""stent with a desirably high degree of longitudinal flexibility that has a large expansion ratio between its low profile and expandable diameters; to provide a stent that does not shorten adversely when it is deployed; and to provide a stent that, when it expands, maintains a relatively uniform distribution of its components and to provide such a stent with sufficient radial strength to provide luminal patency.